1. Field of the Invention
The present invention relates to a method for eliminating scattered .gamma.-ray for collecting an and image of detected .gamma.-irradiated from a radioisotope to a living body and accumulated or deposited in an organ, to form an RI distribution image, and eliminates scattered .gamma.-rays in the living body and scattered .gamma.-rays in a gamma camera from this RI distribution image, by setting a window.
2. Description of the Related Art
Nuclear medical apparatuses have been used to detect a radioisotope given to a living body and accumulated or deposited in an organ using a gamma camera and to form an image of the two-dimensional distribution of the radioisotope for diagnostic use. In this system, scattered .gamma.-rays (scattered gamma-rays) are generated in the living body or in the gamma camera (e.g., collimator or NaI scintillator). Since the scattered .gamma.-rays are not necessary for diagnostic information, they should be eliminated. There are two methods known of eliminating scattered Y-rays from an image acquired by a gamma camera (scintillation camera) or the like as disclosed in J. Nucl. Med. 14; 67-72, 1972, J. Nucl. Med. 25; 490-494, 1984, J. Nucl. Med. 29; 195-202, 1988, and IEEE. Tran. Nucl. Science. NS32. 786-793, 1985.
According to the first method, a window a0 is set for a photoelectric peak Pl in the relation of the amplitude versus an energy spectrum E as shown in FIG. 10 (Prior Art). Images within the window a0 are collected and a window b0 is set for a Compton scattered component C0 at the same time or as the next sequence. Based on a photoelectric image A(x, y) and a scattered-ray image S(x, y) acquired from the windows a0 and b0, a process of A(x, y)-R.S(x, y) is executed as a method of eliminating the scattered rays, where radioisotope is a constant representing a predicted ratio of scattered rays included in a photoelectric absorbing peak P1.
According to the second scattered-ray eliminating method, as scattered .gamma.-rays have a distribution dependent on a position (x, y), they are taken as an image more accurately than in the first method. In the relation of the amplitude versus the energy spectrum E as shown in FIG. 11 (Prior Art), therefore, a window having a sufficiently narrow window width .DELTA.E is sequentially shifted from El to Ep having a peak. At this time, images E(x, y) in individual steps El-Ep are collected and their respective energy images dependent on the .gamma. detected positions, at which .gamma.-rays are detected by the gamma camera, are formed, whereby scattered components for the individual positions are acquired.
Those conventional methods of eliminating scattered rays, however, have the following shortcomings.
The first method eliminates scattered .gamma.-rays using only one value of the predicted constant radioisotope. If the distribution of scattered rays varies depending on .gamma. detected positions, however, this distribution differs from the actual physical phenomena. Therefore, the proper scattered-ray elimination cannot be performed for those positions, thus providing less accurate images.
According to the second method, improving the accuracy in eliminating scattered rays requires making .DELTA.E smaller and collection of a large number of images. This takes a significant amount of time in collecting the images. Further, in order to eliminate nuclide scattered rays having two or more photoelectric peaks position signal, a greater number of images should be collected, thus requiring a longer time for collecting images.